1. Field of the Invention
The present invention relates to a front upper structure of an automotive vehicle, and more specifically to a front upper (e.g., hood and fender) structure of a vehicle body which can effectively absorb an impact energy applied to a front upper body of the automotive vehicle by walker's head (by a head impactor in the case of impact tests) in case the vehicle collides with a walker.
2. Description of Related Art
An example of the hood structure of an automotive vehicle of this sort is disclosed in Japanese Published Unexamined (Kokai) Patent Application No. 61-26682.
In this conventional hood structure, as shown in FIG. 1A, an inner panel 7 is attached to an inner surface (on the side of an engine room 5) of a hood outer panel 3 of a hood 1. The inner panel 7 is composed of a frame body 9 and some inner ribs 11 formed on the inner surface of the frame body 9 to reinforce the frame body 9. The frame body 9 is pivotally supported by a vehicle body via two hood hinges 13 attached on both rear sides of the frame body 9. An engine 15 is disposed within the engine room 5.
Further, as shown in FIG. 1B, a flat plate portion 17 is formed between two of the inner ribs 11 located over the engine 15, and a plurality of punch-off project pieces 19 (formed by punching out the flat plate portion 17 so that some curved pieces 19 can project downward to the engine 15) are formed in the flat plate portion. As shown in FIG. 1C, the cross-sectional shape of each of the punch-off project pieces 19 is formed into roughly circular arc shape and both ends 21 thereof are connected to the flat plate portion 17.
In the hood structure as described above, in case a head of a walker collides with an outer surface of the hood outer panel 3, since the hood outer panel 3 and the inner ribs 11 are deformed into the engine room 5, the punch-out project pieces 19 also collide with the engine 15 and deformed into crush. Accordingly, it is possible to absorb an impact energy applied onto the hood 1 over the engine 15 owing to the deformation of the punch-out project pieces 19, while reducing a movement distance (referred to as a stroke, hereinafter) of the hood 1.
Even if the impact energy can be absorbed by the hood 1 as described above, however, the impact energy applied to the walker's head cannot be necessarily reduced safely in practice.
As the experimental data with respect to the head impact characteristics, a WSTC (Wayne State Tolerance (Curve) as shown in FIG. 2 is so far well known, which is disclosed in [New Automotive Vehicle Engineering Manual &lt;Third Edition&gt;], Published by Japanese Automotive Technology Association on Sep. 30, 1983, pp 2 to 30 and in [Automotive Vehicle Engineering Encyclopedia) &lt;16-volumes&gt;], Published by Sankaido Inc. on Mar. 20, 1980, pp 201 to 203, respectively.
In this WSTC, an effective acceleration G is used as a parameter. This effective acceleration G is an average acceleration obtained by dividing an integral value of acceleration by a duration time ms. Therefore, the effective acceleration corresponds to an average reaction force received by the walker's head in case of collision.
The WSTC shown in FIG. 2 indicates that: even if the average reaction force received by a head impactor (the walker's head in case of collision) is small to some extent (e.g., the effective acceleration=G1, where the duration time is long (e.g., the duration time=T1), the impact energy applied to the head impactor reaches a dangerous region. On the other hand, even if the average reaction force received by the head impactor (the walker's head) is large to some extent (e.g., the effective acceleration=G2), where the duration time is extremely short (e.g., the duration time&lt;T2), the impact energy does not reach the dangerous region but remains in a safe region.
In other words, since the head impact characteristics are determined on the basis of both the acceleration and the duration time, the head impact characteristics are not necessarily reduced even if a large impact energy is absorbed. In other words, there exists such a case that the head impact characteristics can be improved by increasing the initial reaction force applied to the walker's head to some extent within a predetermined short time, without maintaining a relatively small reaction force for a relatively long time.
Further, the WSTC is the experimental data obtained when the applied acceleration is assumed to be linear. In practice, however, since the actual impact energy received by the head impactor in case of collision with the hood is not of linear, but has a rather complicated acceleration waveform, it is impossible to directly apply the WSTC to the actual impact against the walker's head. Therefore, a method of adopting an HIC (Head Injury Criterion) value is so far well known as a method of evaluating the safety on the basis of the WSTC and various impact test results using an impactor.
Here, the HIC value can be derived from the following formulae: ##EQU1## where t1 and t2 denote any duration time (0&lt;t1&lt;t2) during which the acceleration is applied; a(t) denotes an acceleration at a center of gravity of the head. The smaller HIC value is, the higher will be the safety, and the safety limit thereof is determined as HIC=1000, in general.
In accordance with the above formulae, the HIC value can be calculated as the maximum value of those obtained (1) by first obtaining an average acceleration a1.sub.2 between t1 and t2 in the duration time, (2) by raising the obtained average acceleration a.sub.12 to 2.5-th power, and (3) by further multiplying the raised acceleration by the duration time (t2-t1). Therefore, when the impact behavior (the acceleration waveform) differs, the HIC value differs in principle, so that the magnitude of the HIC value can be decided on the basis of the average acceleration a.sub.12 and the duration time (t2-t1). Further, since the relationship between the average acceleration a.sub.12 and the duration time (t2-t1) can be replaced with the relationship between the hood reaction force and the hood stroke (movement distance), it is also possible to decide the magnitude of the HIC value on the basis of the reaction force and the stroke of the hood.
As described above, for the reason that a large amount of the impact energy can be absorbed, the HIC value is not necessarily reduced uniformly; that is, even if the amount of absorbed impact energy is the same, there exists the case where the HIC value differs from each other. Further, even if the HIC value is small at only a single point on the hood, there exists the case where the HIC value is large at the other points on the hood.
Accordingly, in the conventional hood structure as shown in FIG. 1A, since the reaction force for collapsing the punch-out project pieces 19 formed into circular arc shape in cross section will not rise abruptly, there exists such a possibility that a relatively high reaction force is maintained for a relatively long time. Therefore, in order to reduce the head impact characteristics, it is necessary to reduce the collapse reaction force of the punch-out project pieces 19 and further increase the stroke of the hood. In other words, in the conventional hood structure as shown in FIGS. 1A and 1B, it has been necessary to dispose the hood at a relatively high position vertically away from the engine in order to increase the stroke of the hood, thus resulting in another problem in that the front visual field of the automotive vehicle is narrowed.